Large scale battery systems are used as power storage devices for a variety of electric and hybrid electric vehicles. A few examples of vehicles that can be driven with electric or hybrid power are automobiles, boats, and trolley cars. These battery systems typically range in capacity from 10 kWh up to 100 kWh with nominal overall battery voltage ratings from 44.4 Vdc to 444 Vdc.
The most common approach to building large scale battery systems is to connect a number of individual high capacity battery cells together in series and parallel to achieve the desired system voltage and capacity. Each battery system may be assembled from tens to hundreds of individual battery cells. As consequence of the cell manufacturing process, any group of cells may exhibit varying distribution of at least two key parameters: internal impedance and charge holding capacity. The variation in both impedance and capacity within a group of manufactured cells can present a challenge in the manufacture of large scale battery systems, as parallel and series cell combinations within the system experience variations in impedance and capacity. Variation in impedance and capacity within the system directly affects overall battery system characteristics including the following: capacity, energy storage efficiency, cycle lifetime, thermal energy loss, internal temperature uniformity, cost of balancing electronics, and cost of heat removal (cooling) system.